Add arbitrary directives to squid.conf through an attribute

Add arbitrary directives to squid.conf by providing a list of strings in the 'directives' attribute for the cookbook.

Example (in a wrapper cookbook)

    default['squid']['directives'] = [
        'ignore_expect_100 on',
        'acl BrokenButTrustedServers dstdomain brokenserver.com' ,
        'sslproxy_cert_error allow BrokenButTrustedServers',
        'sslproxy_cert_error deny all'
    ]

There's no way for specifying where in squid.conf these directives go;  it was sufficient for me for the directives to be appended to the bottom of squid.conf